The present invention relates to plasma or like display systems based on sub-field gradation display systems and, more particularly, to improvements in the gradation display performance in the display of moving image consisting of intermediate gradations which are obtainable from television signals or the like.
In video or computer terminal displays, the gradation display performance is very important. Among gradation display schemes are those, which are analog controlled as in a cathode-ray tube (CRT). In such a scheme, an input signal voltage is applied without being substantially deformed to a grid for electron beam current control. The light intensity of emission is determined by the magnitude of the current and substantially step-less or continuous control of the gradation display can be performed. Some gradation display schemes for plasma or like display utilize a memory effect. Such a display scheme is essentially a binary-coded display system, and requires a special gradation display method. Gradation display schemes are roughly classified into two types, i.e., analog display type and digital display type. A special method of gradation display which is utilized for plasma or like display, will now be described.
It is conceivable to increase the virtual gradation number by means of dither patterns or error dispersion as in printers, for instance. Such a scheme, however, requires a considerably fine cell structure if a desired gradation number and a desired resolution are both to be obtained, and is therefore not practical so much. A sub-field scheme is adopted as a more general scheme for binary-coded display systems. This scheme is applicable to quick response display systems such as those for plasma display. In the scheme, a video signal is quantized, and one field data thus obtained is displayed for each gradation bit on a time division basis. Specifically, one field period is split into a plurality of fields called sub-fields, which are each weighted by the number of times of light emission corresponding to each gradation bit. Such sub-fields which are obtained by the time division basis method, and are used to reproduce successive images. This scheme has resort to an integrated sight field effect for storing image over one field. Natural intermediate tone image are thus obtainable.
For realizing, for instance, 64-gradation display with this scheme, usually an input analog video signal is first quantized (or A/D converted) to obtain a light intensity signal of 6 bits individually representing successive light intensity gradation data each of double the light intensity level of that of the preceding one. The quantized video signal is stored in a frame buffer memory. Denoting the most significant bit (MSB) representing the highest light intensity by B1 and the successively less significant bits by B2 to B6, the light intensity ratios of the individual bits are 32:16:8:4:2:1. These bits are selected for the individual image elements to obtain a 64-gradation display with light intensity level gradations from level 0 to level 63.
A sub-field scheme display of a discrete scan/sustain discharge drive type, which is utilized in AC color plasma display, will now be briefly described with reference to FIGS. 10(A) to 10(B). One field period is usually set to about 1/60 second, in which no flicker can be perceived, and as shown in FIG. 10(A) it is split into six sub-fields, i.e., a 1-st to a 6-th sub-field SF1 to SF6, each consisting of a scan period and a sustain discharge period.
In the sub-field SF1, data are written in the individual image elements according to display data of the MSB B1 in the scan period. After the data have been written over the entire panel face, a sustain discharge pulse is applied to the entire panel face to cause display by light emission of the image elements, in which the data have been written. The following sub-fields SF2 to SF6 are also driven likewise. To obtain sufficient light intensity during the sustain discharge period of each sub-field, the pulse application for light emission is caused, for instances, 256 times in the sub-field SF1, and 128, 64, 32, 16 and 8 times in the following sub-fields SF2 to SF6. Basically the same sub-field driving is made in the case of merged scan/sustain discharge type driving as shown in FIG. 10(B), or the case of continuous merged scan/sustain discharge type driving over adjacent fields. Such a sub-field drive scheme is adopted because of the necessity of modulating the light intensity of emission with the number of times of light emission or the time thereof. Naturally, high speed performance of scanning and data writing in short periods of time is required to realize a plurality of times of scan in one field period. Recently, data writing performance of plasma display panels has been improved to permit data writing even in 3 microseconds or below, and 8-sub-field 256-gradation full color display has been realized.
A sub-field array constituting one field, in which the light intensity ratios are progressively reduced with time, is called a descending sequence sub-field array. On the other hand, a sub-field array in which the light intensity ratios are progressively increased with time, is called an ascending sequence sub-field array. These sub-field arrays are not special ones but have been usually used. Either sub-field array provides for satisfactory gradation display performance in still image display.
However, the use of such sub-field schemes for moving image display, results in image disturbances or defects in dependence on video. For example, with a motion of a human's face or like object, providing smoothly changing brightness in display on the screen, dark or bright gray scales appear on the image portion which are intrinsically smooth. In color display, such a motion causes generation of color deviation gray scales or scene resolution deterioration.
Specifically, where the above descending sequence sub-field array is adopted, with a motion of a human's face (which is a pattern with darker edges than a central portion), bright gray scales (hereinafter referred to as bright gray scale disturbances), not seen in the original scene, appear and proceed in the direction of the motion, and also dark gray scales (hereinafter referred to as dark gray scale disturbances) appear and proceed in the opposite direction. Where the ascending sequence sub-field array is adopted, bright and dark gray scale disturbances (which are hereinafter referred to as gray scale disturbances of moving images) are caused conversely. In color image display, the gray scale disturbances of moving images appear at different positions with the different colors because of spatially different bit digit raise points thereof. In this case, the disturbances are sometimes called color gray scale disturbances, and essentially they are generated by combinations of bright and dark gray scales in the individual colors of the color image display. This phenomenon is a cause of color deviation, resolution deterioration, etc. in the moving image display.
A CRT can essentially end the display momentarily when displaying a certain gradation level. Besides, analog data is displayed on the CRT screen with electron beam intensity modulation according to the light intensity level. On the other hand, in a plasma display or like system using the sub-field scheme for display, each gradation bit is time-division displayed slowly in a period which is nearly one field, and the viewer synthesizes by the visual sense one frame of image from the individual displayed gradation bit images with an integrating effect of the eyes. In such a state, the viewer can deviate the position of synthesis by the visual sense with his or her will by such a way as horizontally shaking the face or moving the eyesight before completion of one field of image. Moving the eyesight in random timing mostly results in deviation of the position of sub-field synthesis by the visual scene. This means that a moving image display state can be produced with a will in a still image display state. In genuine moving image display, the displayed image itself is moved with time, and motion of the viewer's eyesight is naturally caused without the viewer's will. This leads to frequent failure of completion of one field image in the field image synthesis by the visual sense. Such frequent failure of completion synthesis of one field image is thought to be a principle underlying gray scale disturbances of moving images.
To solve this problem, some schemes have been proposed. Takigawa, "TV Display on AC Plasma Panel", Trans. IECE Japan, '77/Vol. J60-A, No. 1, pp. 56-62, reports that it is effective to optimize the sub-field array such that the mean light intensity over a time corresponding to one field is reduced in error before and after the bit carry-up and -down and that in 5-bit, i.e., 32-gradation, display an adequate sub-field array is such that the light emission time of a more significant bit is provided in a central position. He also reports that it is effective to reduce the display time in one field. He further reports that in experiments, satisfactory display could be realized by providing the light emission time for display in one-fourth of one field and thereby combining this light emission time with the above sub-field array.
Kohgami, in "TV Intermediate Tone Display System Using Memory Type Gas Discharge Panel", EICE Japan, Technical Report, EID 90-9, 1990, reports that the gray scale disturbances can be improved by setting the time interval from the first bit of a field till the last bit of the next field to be within 20 milliseconds, which is the threshold period of merging by human's visual sense. Like the Takigawa's method noted above, it is also reported that the time interval can be set to be within 20 milliseconds for obtaining an improvement in the gray scale disturbances by arranging sub-fields not over the entire field but in a portion of the field on one side thereof. It is further reported that the same condition can be met by arranging a more significant bit of a long light emission time in a split fashion. It is still further reported that, in 8-bit display, the time from the first bit of a field till the last bit of the next field, could be made to be 18.8 milliseconds to improve the gray scale disturbances by splitting the MSB B1 into half sub-fields SF1-1 and SF1-2, splitting the next significant bit B2 into half sub-fields SF2-1 and SF2-2, and forming a sub-field array as one field consisting of 10 sub-fields with the half sub-fields in spaced-apart arrangement such as "SF2-1, SF1-1, SF8, SF7, SF6, SF5, SF4, SF3, SF2-2, SF1-2". In this sub-field array, the hyphenated expression of SF represents half sub-fields, and the numeral after the hyphen represents the order of occurrence in the drive sequence. The non-hyphenated expression of SF represents non-split sub-fields. In the following description, this way of expression is used.
Aside from the above reports, various investigations have been made in order to obtain improvement regarding the gray scale disturbances of moving images with sub-field array optimization. Japanese Laid-Open Patent Publication No. 3-145691 (published on 1991) shows a sub-field array, in which the second and third significant bit sub-fields are arranged on the opposite sides of the MSB sub-field. Japanese Laid-Open Patent Publication No. 7-7702 (published on 1995) shows a sub-field array, in which, unlike the Japanese Laid-Open Patent Publication No. 3-145691 (published on 1991), the second and third significant bit sub-fields are arranged as far apart as possible from the MSB sub-field which is arranged as a central sub-field by arranging sub-fields, which are spaced time-wise from the MSB sub-field, to be adjacent the opposite ends thereof.
The inventors of this invention conducted tests on the above prior art schemes and confirm the effects thereof. It was found that the image quality obtainable with either scheme is insufficient compared to that obtainable with a display using a CRT. For example, the sub-field sequence interchange scheme is not improved so much compared to the simple ascending or descending sequence scheme although it can be readily realized in view of the cost and circuit scale.
Japanese Laid-Open Patent Publication No. 7-175439 shows a scheme, in which the most significant bit sub-field is split into half sub-fields, and a sub-field array of, for instance, "SF8, SF6, SF4, SF1-1, SF2, SF1-2, SF3, SF7" is formed. The same publication also shows splitting the most significant bit sub-field into quarter sub-fields, splitting the second significant bit sub-field into half sub-fields, and forming a sub-field array of, for instance, "SF8, SSP6, SF1-1, SF4, SF2-1, SF1-2, SF3, SF1-3, SF2-2, SF5, SF1-4, SF7". The latter sub-field scheme requires 12 sub-fields. The former sub-field scheme is a generally conceivable one. However, the effect obtainable by splitting the sole most significant bit sub-field is insufficient. Rather, it was confirmed that there are better sub-field arrays than the one with the above limitation. With the latter scheme, it was confirmed that the obtainable effects less in spite of using as many as 12 sub-fields and that it is impossible to obtain sufficient performance even by considerably reducing the time required for the sub-field as a whole.
In the meantime, coding schemes for binary coding with redundancy, other than conventional pure binary coding, have been proposed. For example, Toda et al, "A Modified-Binary-Coded Light-Emission Scheme for Suppressing Cray Scale Disturbances of Moving Images", ASIA DISPLAY '95, S19-9, shows a scheme for suppressing the gray scale disturbances of moving images by making bit carry points unclear with binary coding with redundancy. In this scheme, sub-fields corresponding the most and second significant bits in a usual binary code are each split into half sub-fields, and these four half sub-fields are reconstituted into those of the same weight. Specifically, usually the most and second significant bits with respective weights of 128 and 64 are split into respective weights of 64 and 32, and these four pieces of data are converted to those of the same weight of 48. With this arrangement, it is possible to disperse the bit carry points, which conventionally concentratedly took place at a particular place, to several places. However, this scheme requires dealing with 10-bit data to obtain an accuracy of 8 bits. When it is considered as an actual system, the scheme is not always advantageous because of increases of the cost and circuit scale.
Japanese Laid-Open Patent Publication No. 7-271325 shows a scheme, in which a sub-field array provided for display is bit-by-bit controlled by utilizing coding with redundancy to provide a checkered pattern with alternate appearance of bright and dark gray scale disturbances of moving images. The principle underlying this scheme is to make visual image disturbances less noticeable by utilizing the resolution limit of the eyes, which is imposed when viewing the display panel face from a distant position. Although this scheme is effective, it is subject to bit-by-bit eyesore-like disturbances, which can be noticed when a display involving motion is viewed very carefully. Besides, the use of codes with redundancy results in an actual gradation number reduction compared to the case of perfectly binary codes with the same number of bits used for the display.
The inventors of the present invention further conducted tests concerning the relation between the "threshold merging period" which has heretofore been a fixed concept and gray scales in moving images. A first conceivable scheme is to reduce the sub-field time itself in a sense of providing for an operation close to that of the CRT. This scheme could be tested by providing certain operational conditions. In contrast to the conventional intelligence, gray scale disturbances of moving images could not be sufficiently suppressed from the standpoint of the high display quality even by setting the entire drive sequence time within a considerably short period (of about 4 ms), although this scheme was considerably superior to the purely ascending or descending sequence scheme. With a scheme in which a large number of sub-fields are split like the latter scheme shown in the Japanese Laid-Open Patent Publication No. 7-175439 (published on 1995), the effect which is obtainable by merely splitting sub-fields and adequately arranging the resultant half sub-fields, was insufficient even with a drive sequence time set to meet the above threshold merging period condition.
The typical schemes shown as measures against the gray scale disturbances of moving images in the above literatures and Japanese Laid-Open patent publications, note and cope with more significant bits which are attributable to greater gray scale disturbances of moving images. Certainly, by providing measures with respect to more significant bits, a drastic effect of improvement is obtainable compared to the case when no measure is provided. However, while the conventional measures permit reduction or elimination of great gray scale disturbances of moving images, the effect of improvement can be obtained only up to a certain level. That is, no effect is obtainable with respect to relatively low level disturbances which result in image quality deterioration. The inventors of the present invention could obtain a recognition that it is necessary to take even the disturbances attributable to less significant bits into considerations in order to realize high quality display of moving images.